The invention relates generally to the control of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x) in the exhaust of internal combustion engines. More particularly, the invention relates to the removal of NO.sub.x when the exhaust gases include excess oxygen beyond that needed for combustion of the fuel. This is typically the case with lean-burn engines, diesel engines, and other engines currently under development which produce such exhaust gases.
In recent years three-way catalysts have been used to remove all of the three principal noxious components in auto exhaust gases. The engines are run with stoichiometric air/fuel ratios and the catalysts are able to remove all three components at the same time, that is, a single catalyst is sufficient over the range of engine operating temperatures. More recently development of so-called "lean-burn" engines is being driven by the desire to improve fuel economy. However, such engines operate with air-fuel ratios which are far from the typical stoichiometric conditions. Instead of an air-fuel ratio of about 14.55/1 by weight, the lean-burn engine may operate with air-fuel ratios above 18/1, up to about 22-24/1, or even higher ratios for diesel engines. Under such conditions the engine exhaust will include more hydrocarbons, less carbon monoxide, and less, but still excessive nitrogen oxides. While an oxidation catalyst is capable of removing hydrocarbons and carbon monoxide, since the oxygen content is high, say about 3-10% by volume, it is clear that conditions are not favorable for the reduction of nitrogen oxides. Much effort has gone into a search for catalysts which can destroy nitrogen oxides under oxidizing conditions, but the results have not been satisfactory to date. Published information most pertinent to the present invention will be discussed below.
The use of base metals, particularly copper, ion-exchanged onto a zeolite support has been proposed by many workers in the art to be effective for reduction of nitrogen oxides. Others have suggested that such a catalyst should be combined with oxidation or three-way catalysts, usually in sequence. Toyota has proposed such arrangements in applications published in Japan. In JP Kokai 310742/1989 reference is made to earlier applications in which zeolites carrying transition metals are combined with downstream three-way or oxidation catalysts. These were said to be deficient and a catalyst was proposed which added noble metals to a copper-zeolite catalyst. Various methods of combining these materials are suggested. A related application is JP Kokai 127044/1989 in which an oxidation catalyst is deposited as a first layer, followed by a second layer of copper on a zeolite.
In EP 0488250A1 Toyota proposed three catalysts in series. Toyota discusses the use of various catalysts for removal of NO.sub.x from the exhaust of lean burn engines. Pt on zeolite is useful at low temperatures and Cu on zeolite at higher temperatures, but other catalysts are shown to be useful at inlet temperatures of about 300.degree. C. These are the noble metals combined with the oxides of rare earth metals and metals from Group IVa of the Periodic Table (IUPAC), such as Ti, Zr, and Hf.
In EP 0494388A1 the applicants disclose two stages of catalysts for first removing nitrogen oxides and then oxidizing the remaining hydrocarbons and carbon monoxide. The NO.sub.x removal catalyst are defined as phosphates, sulfates, or aluminates of transition metals of the 4th period of the Periodic table (e.g. Cr, Mn, Fe, Co, Ni, Cu, Zr). The oxidation catalyst is generally described as a noble metal, a base metal or a perovskite on a support.
There are two generally recognized routes to removing nitrogen oxides. First, the nitrogen oxides can be decomposed to the diatomic nitrogen and diatomic oxygen. This reaction is thermodynamically favored, but catalysts which are able to carry out this reaction under the highly oxidizing conditions and high temperatures found in engine exhaust have not yet been found. The second route is the chemical reduction of nitrogen oxides using reducing agents present in the exhaust, such as carbon monoxide, hydrocarbons, and hydrogen. This is considered to be the mechanism of the three-way catalyst. However, such catalysts see the exhaust from an engine operating with a stoichiometric air-fuel ratio which contains little oxygen. When a large excess of oxygen is present, as in a lean-burn or diesel engine, the oxygen can preferentially react with the hydrocarbons, carbon monoxide, and hydrogen thus removing the reducing agents needed to remove nitrogen oxides. The present inventors have found that this problem can be overcome, as will be disclosed in the discussion below.